Ultraviolet (UV)-emitting phosphors are used in fluorescent lamp applications for skin tanning where both UVA and UVB radiation is needed. UVA is defined by the U.S. Food & Drug administration (FDA) as radiation from 320 nm to 400 nm and UVB is defined as radiation from 260 nm to 320 nm. The allowable levels of UVB radiation emitted by suntan lamps is tightly controlled and the use of various UV-emitting phosphor blends to control the proportions of UVA to UVB radiation is well known. Examples of such UV-emitting blends are disclosed in U.S. Pat. Nos. 4,959,551, 4,967,090, 5,216,323, 4,499,403, and 4,933,600.
A cerium-activated strontium magnesium aluminate phosphor, Sr(Al,Mg)12O19:Ce, is commonly used in suntan lamp applications to increase the amount of UVB radiation emitted by the lamp in order to improve the suntanning process. (See, e.g., U.S. Pat. Nos. 5,234,710 and 6,984,931). Typically, the amount of the Sr(Al,Mg)12O19:Ce phosphor ranges between 6% to 8% by weight (wt. %) of the phosphor blend, but it may comprise up to about 12 wt. % in some cases. The greater portion of the phosphor blend in suntan lamps emits UVA radiation and may comprise one or more of the known UVA-emitting phosphors such as BaSi2O5:Pb, SrB4O7:Eu, or YPO4:Ce.
Suntan lamps are based on the conventional low-pressure mercury discharge prevalent in fluorescent lighting. The primary UV excitation energy emitted by a low-pressure mercury discharge is at 254 nm. However, a sizeable portion of the total UV radiation is also emitted at 185 nm. The amount of 185 nm radiation depends upon the fill gas composition and pressure of the lamp, but this can be as much as 8-10% of the total UV radiation. Unfortunately, the conventional Sr(Al,Mg)12O19:Ce phosphor is poorly excited by the 185 nm radiation, so that this energy is wasted with respect to this phosphor.